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自动释放池源码分析

自动释放池源码分析

作者: yxibng | 来源:发表于2022-01-25 18:25 被阅读0次

    autoreleasepool

    参考:Using Autorelease Pool Blocks

    Autorelease pool blocks provide a mechanism whereby you can relinquish ownership of an object, but avoid the possibility of it being deallocated immediately (such as when you return an object from a method).

    翻译一下就是 Autorelease Pool 对管理的对象提供延迟释放机制。

    形式

    @autoreleasepool {
        // Code that creates autoreleased objects.
    }
    //可以被嵌套
    @autoreleasepool {
        // . . .
        @autoreleasepool {
            // . . .
        }
        . . .
    }
    

    autoreleasepool 分析

    @autoreleasepool

    main.m

    #import <Foundation/Foundation.h>
    
    int main(int argc, const char * argv[]) {
        @autoreleasepool {
            // insert code here...
            NSLog(@"Hello, World!");
        }
        return 0;
    }
    

    使用clang 改写成c++ 代码

    clang -rewrite-objc  main.m
    

    目录下面会生成一个 main.cpp, 拉到最下面

    int main(int argc, const char * argv[]) {
        /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
    
            NSLog((NSString *)&__NSConstantStringImpl__var_folders_67_5cysdnrn6_dfz1lp4rknxkhh0000gp_T_main_0b8b3d_mi_0);
        }
        return 0;
    }
    

    看到autoreleasepool转换成了

    { __AtAutoreleasePool __autoreleasepool; 
    //...
    }
    

    找到__AtAutoreleasePool的定义

    extern "C" __declspec(dllimport) void * objc_autoreleasePoolPush(void);
    extern "C" __declspec(dllimport) void objc_autoreleasePoolPop(void *);
    struct __AtAutoreleasePool {
      __AtAutoreleasePool() {atautoreleasepoolobj = objc_autoreleasePoolPush();}
      ~__AtAutoreleasePool() {objc_autoreleasePoolPop(atautoreleasepoolobj);}
      void * atautoreleasepoolobj;
    };
    

    __AtAutoreleasePool是一个结构体, 构造函数调用objc_autoreleasePoolPush
    析构的时候调用objc_autoreleasePoolPop

    {}包裹的作用域里面创建的对象会被加入到该autoreleasepool中管理,
    作用域结束,__autoreleasepool销毁,调用析构函数,对管理的对象发送release消息。

    相当于

    {   
        //创建一个自动释放池
        atautoreleasepoolobj = objc_autoreleasePoolPush();
     
        //...添加对象到自动释放池
    
        //弹出自动释放池,对其中的对象发送 release 消息
        objc_autoreleasePoolPop(atautoreleasepoolobj) //
    }
    

    objc_autoreleasePoolPush

    extern "C" __declspec(dllimport) void * objc_autoreleasePoolPush(void);
    extern "C" __declspec(dllimport) void objc_autoreleasePoolPop(void *);
    
    

    该方法在runtime源码中可以找到,位于NSObject.mm

    void *objc_autoreleasePoolPush(void)
    {
        return AutoreleasePoolPage::push();
    }
    
    void objc_autoreleasePoolPop(void *ctxt)
    {
        AutoreleasePoolPage::pop(ctxt);
    }
    

    AutoreleasePoolPage 数据结构

    结论: AutoreleasePoolPage 的结构是双向链表


    b770ab80.png
    class AutoreleasePoolPage : private AutoreleasePoolPageData
    {
        static size_t const SIZE = 4096; //4KB
    private:
        //tls 对应的key
        static pthread_key_t const key = AUTORELEASE_POOL_KEY;
        static uint8_t const SCRIBBLE = 0xA3;  // 0xA3A3A3A3 after releasing
        static size_t const COUNT = SIZE / sizeof(id);
        static size_t const MAX_FAULTS = 2;
    
        // EMPTY_POOL_PLACEHOLDER is stored in TLS when exactly one pool is 
        // pushed and it has never contained any objects. This saves memory 
        // when the top level (i.e. libdispatch) pushes and pops pools but 
        // never uses them.
    #   define EMPTY_POOL_PLACEHOLDER ((id*)1)
    //  auto release pool 哨兵
    #   define POOL_BOUNDARY nil
    }
    

    static成员不占用实例对象的内存空间,所以每个AutoreleasePoolPage实例的大小为56字节,
    详见下面的分析。
    AutoreleasePoolPage 是一个c++的类, 这个类继承自 AutoreleasePoolPageData

    AutoreleasePoolPageData

    struct AutoreleasePoolPageData
    {
        magic_t const magic; //16
        __unsafe_unretained id *next; //8
        pthread_t const thread; //8
        AutoreleasePoolPage * const parent; //8
        AutoreleasePoolPage *child;//8
        uint32_t const depth;//4
        uint32_t hiwat;//4
    };
    
    

    AutoreleasePoolPageData 的实例大小为 56 字节,AutoreleasePoolPage 类并没有添加新的成员变量,
    因此实例的大小和父类的实例大小一致,都是 56 字节。

    magic_t
    struct magic_t {
        static const uint32_t M0 = 0xA1A1A1A1;
    #   define M1 "AUTORELEASE!"
        static const size_t M1_len = 12;
        uint32_t m[4];//16 bytes
    
        magic_t() {
            ASSERT(M1_len == strlen(M1));
            ASSERT(M1_len == 3 * sizeof(m[1]));
    
            m[0] = M0;
            strncpy((char *)&m[1], M1, M1_len);
        }
    
        ~magic_t() {
            // Clear magic before deallocation.
            // This prevents some false positives in memory debugging tools.
            // fixme semantically this should be memset_s(), but the
            // compiler doesn't optimize that at all (rdar://44856676).
            volatile uint64_t *p = (volatile uint64_t *)m;
            p[0] = 0; p[1] = 0;
        }
    
        bool check() const {
            return (m[0] == M0 && 0 == strncmp((char *)&m[1], M1, M1_len));
        }
    
        bool fastcheck() const {
    #if CHECK_AUTORELEASEPOOL
            return check();
    #else
            return (m[0] == M0);
    #endif
        }
    
    #   undef M1
    };
    

    用于校验自动释放池是否被被损坏

    • 构造成功
    m[0]= 0xA1A1A1A1 
    m[1-3] = AUTORELEASE!
    
    • 检查的时候
      判断第一个字节是0xA1A1A1A1, 后面三个字节是AUTORELEASE!

    • 析构的时候

    m[0] = 0 
    m[1-3] = 0
    
    next

    next 用于指向当前页的栈顶指针

    thread

    thread 保存了当前page所对应的线程

    parent

    当前节点的父节点

    child

    当前节点的子节点

    AutoreleasePoolPage::push

    static inline void *push() 
    {
        id *dest;
        if (slowpath(DebugPoolAllocation)) {
            // Each autorelease pool starts on a new pool page.
            dest = autoreleaseNewPage(POOL_BOUNDARY);
        } else {
            dest = autoreleaseFast(POOL_BOUNDARY);
        }
        ASSERT(dest == EMPTY_POOL_PLACEHOLDER || *dest == POOL_BOUNDARY);
        return dest;
    }
    

    如果DebugPoolAllocation为true
    执行autoreleaseNewPage
    否则autoreleaseFast

    slowpath(DebugPoolAllocation) 告诉编译器,预测该条件大概率为false

    if (maybeMallocDebugging) {
        const char *insert = getenv("DYLD_INSERT_LIBRARIES");
        const char *zombie = getenv("NSZombiesEnabled");
        const char *pooldebug = getenv("OBJC_DEBUG_POOL_ALLOCATION");
        if ((getenv("MallocStackLogging")
                || getenv("MallocStackLoggingNoCompact")
                || (zombie && (*zombie == 'Y' || *zombie == 'y'))
                || (insert && strstr(insert, "libgmalloc")))
            &&
            (!pooldebug || 0 == strcmp(pooldebug, "YES")))
        {
            DebugPoolAllocation = true;
        }
    }
    

    如果是debug内存问题,开启以上选项的时候,每次都将创建新的page

    大多数情况下执行的都是autoreleaseFast

    autoreleaseNewPage
    id *autoreleaseNewPage(id obj)
    {
        AutoreleasePoolPage *page = hotPage();
        if (page) return autoreleaseFullPage(obj, page);
        else return autoreleaseNoPage(obj);
    }
    
    hotPage
    static inline AutoreleasePoolPage *hotPage() 
    {
        AutoreleasePoolPage *result = (AutoreleasePoolPage *)
            tls_get_direct(key);
        if ((id *)result == EMPTY_POOL_PLACEHOLDER) return nil;
        if (result) result->fastcheck();
        return result;
    }
    

    hotPage 获取当前正在使用的page。从当前线程的tls(thread local storage)中获取
    key为static pthread_key_t const key = AUTORELEASE_POOL_KEY;
    的值,对应的就是当前的hotpage,可能为空。

    关于 EMPTY_POOL_PLACEHOLDER

        // EMPTY_POOL_PLACEHOLDER is stored in TLS when exactly one pool is 
        // pushed and it has never contained any objects. This saves memory 
        // when the top level (i.e. libdispatch) pushes and pops pools but 
        // never uses them.
    #   define EMPTY_POOL_PLACEHOLDER ((id*)1)
    

    验证

    #import <Foundation/Foundation.h>
    #import <pthread.h>
    int main(int argc, const char * argv[]) {
        @autoreleasepool {
            dispatch_group_t group = dispatch_group_create();
            dispatch_group_enter(group);
            dispatch_async(dispatch_get_global_queue(0, 0), ^{
                pthread_key_t key = 43;
                uint64_t val = (uint64_t)pthread_getspecific(key);
                NSLog(@"%llu",val);
                dispatch_group_leave(group);
            });
            dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
        }
        return 0;
    }
    

    输出 val = 1, 而EMPTY_POOL_PLACEHOLDER刚好等于1.
    分析: gcd启动了新的线程,push了一个占位的autoreleasepool,在去添加对象的时候,才会创建真正的自动释放池page。
    如果没有添加对象,创建了自动释放池page(4KB),会造成内存浪费。这是一个优化点。

    为什么key是43 ?

    objc-os.h

    #if defined(__PTK_FRAMEWORK_OBJC_KEY0)
    #   define SUPPORT_DIRECT_THREAD_KEYS 1
    #   define TLS_DIRECT_KEY        ((tls_key_t)__PTK_FRAMEWORK_OBJC_KEY0)
    #   define SYNC_DATA_DIRECT_KEY  ((tls_key_t)__PTK_FRAMEWORK_OBJC_KEY1)
    #   define SYNC_COUNT_DIRECT_KEY ((tls_key_t)__PTK_FRAMEWORK_OBJC_KEY2)
    #   define AUTORELEASE_POOL_KEY  ((tls_key_t)__PTK_FRAMEWORK_OBJC_KEY3)
    # if SUPPORT_RETURN_AUTORELEASE
    #   define RETURN_DISPOSITION_KEY ((tls_key_t)__PTK_FRAMEWORK_OBJC_KEY4)
    # endif
    #else
    #   define SUPPORT_DIRECT_THREAD_KEYS 0
    #endif
    
    

    因此 AUTORELEASE_POOL_KEY 实际对应 __PTK_FRAMEWORK_OBJC_KEY3
    __PTK_FRAMEWORK_OBJC_KEY3

    tsd_private.h

    /* Keys 40-49 for Objective-C runtime usage */
    #define __PTK_FRAMEWORK_OBJC_KEY0   40
    #define __PTK_FRAMEWORK_OBJC_KEY1   41
    #define __PTK_FRAMEWORK_OBJC_KEY2   42
    #define __PTK_FRAMEWORK_OBJC_KEY3   43
    #define __PTK_FRAMEWORK_OBJC_KEY4   44
    #define __PTK_FRAMEWORK_OBJC_KEY5   45
    #define __PTK_FRAMEWORK_OBJC_KEY6   46
    #define __PTK_FRAMEWORK_OBJC_KEY7   47
    #define __PTK_FRAMEWORK_OBJC_KEY8   48
    #define __PTK_FRAMEWORK_OBJC_KEY9   49
    
    autoreleaseFast
    static inline id *autoreleaseFast(id obj)
    {
        //如果obj 是nil,会添加一个哨兵
        //obj不为nil,添加对象到自动释放池
        //取得当前hotpage
        AutoreleasePoolPage *page = hotPage();
        if (page && !page->full()) {
            //存在,并且没有满,添加obj到hotpage
            return page->add(obj);
        } else if (page) {
            //存在,已满,创建新的page,父节点为当前page
            return autoreleaseFullPage(obj, page);
        } else {
            //不存在,创建page,设置为hotpage,添加obj到创建的page
            return autoreleaseNoPage(obj);
        }
    }
    
    autoreleaseFullPage
    id *autoreleaseFullPage(id obj, AutoreleasePoolPage *page)
    {
        // The hot page is full. 
        // Step to the next non-full page, adding a new page if necessary.
        // Then add the object to that page.
        ASSERT(page == hotPage());
        ASSERT(page->full()  ||  DebugPoolAllocation);
        /*
        以当前page为父节点,顺着子节点往下,找到第一个还未满的的page
        找不到,创建新的page
        设置hotpage
        添加obj到hotpage
        */
        do {
            if (page->child) page = page->child;
            else page = new AutoreleasePoolPage(page);
        } while (page->full());
    
        setHotPage(page);
        return page->add(obj);
    }
    
    autoreleaseNoPage
    id *autoreleaseNoPage(id obj)
    {
        // "No page" could mean no pool has been pushed
        // or an empty placeholder pool has been pushed and has no contents yet
    
        /*
        没有page 代表
        1. 没有pool被push, hotpage == nil
        2. 有一个占位的pool被push,里面没有任何内容
        */
        ASSERT(!hotPage());
    
        bool pushExtraBoundary = false;
        if (haveEmptyPoolPlaceholder()) {
            /*
            tls对应key为AUTORELEASE_POOL_KEY的value为1
            此时创建page,需要在page中添加哨兵对象
            */
            // We are pushing a second pool over the empty placeholder pool
            // or pushing the first object into the empty placeholder pool.
            // Before doing that, push a pool boundary on behalf of the pool 
            // that is currently represented by the empty placeholder.
            pushExtraBoundary = true;
        }
        else if (obj != POOL_BOUNDARY  &&  DebugMissingPools) {
            /*
             没有hotpage, 需要创建page, 添加的第一个obj应该是哨兵对象
             否则就出错了
            */
            // We are pushing an object with no pool in place, 
            // and no-pool debugging was requested by environment.
            _objc_inform("MISSING POOLS: (%p) Object %p of class %s "
                            "autoreleased with no pool in place - "
                            "just leaking - break on "
                            "objc_autoreleaseNoPool() to debug", 
                            objc_thread_self(), (void*)obj, object_getClassName(obj));
            objc_autoreleaseNoPool(obj);
            return nil;
        }
        else if (obj == POOL_BOUNDARY  &&  !DebugPoolAllocation) {
            /*
            没有hotpage, 没有占位pool page
            此时添加哨兵,直接在tls里保存一个占位的poolpage
            等待添加对象的时候,再去真正创建pool page
            */
            
            // We are pushing a pool with no pool in place,
            // and alloc-per-pool debugging was not requested.
            // Install and return the empty pool placeholder.
            return setEmptyPoolPlaceholder();
        }
    
        // We are pushing an object or a non-placeholder'd pool.
        // Install the first page.
    
        /*
        hotpage 不存在,创建第一个page,父节点是nil
        设置为hotpage
        将obj添加到该page
        */
        AutoreleasePoolPage *page = new AutoreleasePoolPage(nil);
        setHotPage(page);
        
        // Push a boundary on behalf of the previously-placeholder'd pool.
        if (pushExtraBoundary) {
            page->add(POOL_BOUNDARY);
        }
        
        // Push the requested object or pool.
        return page->add(obj);
    }
    
    AutoreleasePoolPage 创建

    构造函数

    AutoreleasePoolPage(AutoreleasePoolPage *newParent) :
        AutoreleasePoolPageData(begin(),
                                objc_thread_self(),
                                newParent,
                                newParent ? 1+newParent->depth : 0,
                                newParent ? newParent->hiwat : 0)
    {
        if (objc::PageCountWarning != -1) {
            checkTooMuchAutorelease();
        }
    
        if (parent) {
            parent->check();
            ASSERT(!parent->child);
            parent->unprotect();
            parent->child = this;
            parent->protect();
        }
        protect();
    }
    

    父类的构造

    AutoreleasePoolPageData(__unsafe_unretained id* _next, pthread_t _thread, AutoreleasePoolPage* _parent, uint32_t _depth, uint32_t _hiwat)
        : magic(), next(_next), thread(_thread),
            parent(_parent), child(nil),
            depth(_depth), hiwat(_hiwat)
    {
    }
    

    关键参数:

    1. 父节点(双向链表,指向父节点的指针)
    2. next指针(向page中添加对象,指向下一个可以添加的位置)
    3. pthread_t 当前线程id
    4. depth 和 hiwat 暂时不讨论

    AutoreleasePoolPage 内存分配

    //SIZE = 4096 = 4KB
    static void * operator new(size_t size) {
        return malloc_zone_memalign(malloc_default_zone(), SIZE, SIZE);
    }
    static void operator delete(void * p) {
        return free(p);
    }
    

    AutoreleasePoolPage 重载了new运算符,为实例分配了4KB的内存。
    其中 56Byte是AutoreleasePoolPage实例的大小,剩下的4040Byte用来存储被管理的对象。
    被管理的对象的类型是id*, 64位下占8Byte, 因此一个page最多可以存储505个对象。

    //添加对象起始地址
    id * begin() {
        return (id *) ((uint8_t *)this+sizeof(*this));
    }
    //添加对象结束地址
    id * end() {
        return (id *) ((uint8_t *)this+SIZE);
    }
    //空page,未管理任何对象
    bool empty() {
        return next == begin();
    }
    //page已满, next == end
    bool full() { 
        return next == end();
    }
    //不满一半
    bool lessThanHalfFull() {
        return (next - begin() < (end() - begin()) / 2);
    }
    

    AutoreleasePoolPage 存在于堆区,前面56个字节存当前实例this,后面4040个字节存管理的对象。


    761bbd9f.png

    begin指向于56字节处, end 指向于 4096字节。
    next指向下一个空闲的位置,当next = end,表示page满了。

    在一次push中

    1. 首先插入哨兵对象,将next++
    2. 将对象添加到next位置,next++
    3. 如果当前页满了,创建新page,继续添加对象。
    4. 因此会产生跨页的自动释放池。
    static inline id autorelease(id obj)
    {
        ASSERT(!obj->isTaggedPointerOrNil());
        //调用autoreleaseFast添加对象
        id *dest __unused = autoreleaseFast(obj);
        return obj;
    }
    

    因此添加相当于

    atautoreleasepoolobj = objc_autoreleasePoolPush();
    //添加对象到自动释放池
    autorelease(obj);
    autorelease(obj);
    autorelease(obj);
        ...
    //弹出自动释放池,对其中的对象发送 release 消息
    objc_autoreleasePoolPop(atautoreleasepoolobj)
    
    add
    id *add(id obj)
    {
        id *ret = next;  
        *next++ = obj;
        return ret;
    }
    

    将obj保存在next指向的位置,next++。
    返回obj存储的地址。

    因此

    atautoreleasepoolobj = objc_autoreleasePoolPush();
    objc_autoreleasePoolPop(atautoreleasepoolobj)
    

    atautoreleasepoolobj 就是当前自动释放池push进的哨兵对象所指的位置,
    当pop的时候,遇到该哨兵对象就停止出栈。

    AutoreleasePoolPage::pop(ctxt)

    static inline void
    pop(void *token)
    {
        AutoreleasePoolPage *page;
        id *stop;
        if (token == (void*)EMPTY_POOL_PLACEHOLDER) {
            // Popping the top-level placeholder pool.
            page = hotPage();
            if (!page) {
                // Pool was never used. Clear the placeholder.
                return setHotPage(nil);
            }
            // Pool was used. Pop its contents normally.
            // Pool pages remain allocated for re-use as usual.
            page = coldPage();
            token = page->begin();
        } else {
            page = pageForPointer(token);
        }
    
        stop = (id *)token;
        if (*stop != POOL_BOUNDARY) {
            if (stop == page->begin()  &&  !page->parent) {
                // Start of coldest page may correctly not be POOL_BOUNDARY:
                // 1. top-level pool is popped, leaving the cold page in place
                // 2. an object is autoreleased with no pool
            } else {
                // Error. For bincompat purposes this is not 
                // fatal in executables built with old SDKs.
                return badPop(token);
            }
        }
    
        if (slowpath(PrintPoolHiwat || DebugPoolAllocation || DebugMissingPools)) {
            return popPageDebug(token, page, stop);
        }
    
        return popPage<false>(token, page, stop);
    }
    

    简化一下就是

    1. 找到token 对应的 page
    2. 调用popPage方法
    static inline void pop(void *token)
    {
        AutoreleasePoolPage *page = pageForPointer(token);
        return popPage<false>(token, page, stop);
    }
    
    pageForPointer
    static AutoreleasePoolPage *pageForPointer(uintptr_t p) 
    {
        AutoreleasePoolPage *result;
        //哨兵地址于page基地址的偏移
        uintptr_t offset = p % SIZE;
        ASSERT(offset >= sizeof(AutoreleasePoolPage));
        //得到page的基地址
        result = (AutoreleasePoolPage *)(p - offset);
        //检查magic字段,判断page是否被破坏
        result->fastcheck();
        //返回找到的page
        return result;
    }
    

    指针p 指向于 pop 时的哨兵对象。 pop 时,哨兵所在的page地址。
    SIZE = 4096
    通过计算uintptr_t offset = p % SIZE 得到哨兵基于page基地址的偏移
    p - offset得到page基地址。
    可以直接求余的原因是每个page都是4096大小,以4096在内存中对齐。

    popPage
    static void
    popPage(void *token, AutoreleasePoolPage *page, id *stop)
    {
        if (allowDebug && PrintPoolHiwat) printHiwat();
    
        page->releaseUntil(stop);
    
        // memory: delete empty children
        if (allowDebug && DebugPoolAllocation  &&  page->empty()) {
            // special case: delete everything during page-per-pool debugging
            AutoreleasePoolPage *parent = page->parent;
            page->kill();
            setHotPage(parent);
        } else if (allowDebug && DebugMissingPools  &&  page->empty()  &&  !page->parent) {
            // special case: delete everything for pop(top)
            // when debugging missing autorelease pools
            page->kill();
            setHotPage(nil);
        } else if (page->child) {
            // hysteresis: keep one empty child if page is more than half full
            if (page->lessThanHalfFull()) {
                page->child->kill();
            }
            else if (page->child->child) {
                page->child->child->kill();
            }
        }
    }
    

    简化一下

    static void popPage(void *token, AutoreleasePoolPage *page, id *stop)
    {
        //将当前自动释放池出栈,可能会跨页面
        page->releaseUntil(stop);
        //将空页面销毁
        if (page->child) {
            // hysteresis: keep one empty child if page is more than half full
            if (page->lessThanHalfFull()) {
                //如果当前页面不满一半被使用,将子page销毁
                page->child->kill();
            }
            else if (page->child->child) {
                //否则将page->child->child销毁掉,保留一个子节点
                page->child->child->kill();
            }
        }
    }
    

    releaseUntil 方法分析

    void releaseUntil(id *stop) 
    {
        /*
        存在跨页面的自动释放,从链表的尾部开始出栈,一直到当前页面
        遇到stop标记的时候停止
        */
        while (this->next != stop) {
            //找到hotpage,跨页面的话就是链表的尾部
            AutoreleasePoolPage *page = hotPage();
            // 找到第一个不为空的page,设置为hotpage
            while (page->empty()) {
                page = page->parent;
                setHotPage(page);
            }
            //保存出栈对象
            id obj = *--page->next;
            //page->next指向的内存改写为0xA3A3A3A3
            memset((void*)page->next, SCRIBBLE, sizeof(*page->next));
            if (obj != POOL_BOUNDARY) {
                //给obj发送release消息
                objc_release(obj);
            }
        }
        //当前页面出栈到stop标志位,设置当前页为hotpage
        setHotPage(this);
    }
    

    kill 方法分析

    void kill() 
    {
        //找到链表的末尾
        AutoreleasePoolPage *page = this;
        while (page->child) page = page->child;
        //从链表末尾开始删除节点
        //do..while,可能删除的是当前节点
        AutoreleasePoolPage *deathptr;
        do {
            //找到子节点
            deathptr = page;
            //找到子节点的父节点
            page = page->parent;
            if (page) {
                //将父节点的child 置空
                page->child = nil;
            }
            //销毁子节点
            delete deathptr;
        } while (deathptr != this);
    }
    

    参考:
    自动释放池的前世今生 ---- 深入解析 autoreleasepool

    Using Autorelease Pool Blocks

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